1. Field of the invention
The present invention relates generally to a system and method for controlling ignition timing for an internal combustion engine applicable to an automotive vehicle and particularly relates to the system and method which reduce vibrations (so-called, shuddering vibrations to be described later) along with rotational vibrations of engines at a time of transient state.
2. Background of the art
Recently, higher performance requirements for vehicle mounted engines are increased so that higher engine driveability, lower fuel consumptions, and higher running characteristics are achieved at higher levels.
Particularly, it is desirable to suppress vibrations of the engine generated due to engine rotational speed variations for the respective cylinders from a driveability standpoint of view.
That is to say, the engine revolutional speed does not smoothly rise under a transient state such as during an acceleration and/or deceleration but involve engine speed variations. Such engine speed variations are transmitted to a drive system of the vehicle in a form of torque variations, thereby generating low-frequency vibrations (hereinafter, referred to as surge) in front and rear directions of the vehicle.
This surge gives the driver an unpleasant feeling of shuddering (so-called shuddering vibrations) so that the engine driveability becomes worsened.
To prevent such shuddering vibrations, an ignition timing controlling system has been proposed in which an ignition timing is controlled so as to suppress engine revolutional speed variations. For example, when the engine revolutional speed is reduced during the generation of shuddering vibrations, the ignition timing is advanced to increase the engine torque. When the engine speed is increased, the ignition timing is retarded to decrease the engine torque. Consequently, the engine speed and torque variations are suppressed.
However, the level at which the surge and shuddering vibrations are suppressed is still insufficient. That is to say, the generation of the shuddering vibration is changed according to a load state of the engine. For example, with a throttle valve widely opened, an engine torque is largely changed when the ignition timing is corrected. On the other hand, in a case of a partial load, the engine torque does not remarkably change even when the ignition timing is corrected.
Another previously proposed ignition timing controlling system has been proposed in which the ignition timing is corrected, with the load state of the engine taken into account, so as to further suppress the generation of shuddering vibrations. (Refer to a Japanese Patent Application First Publication (Non-Examined) Hei 1-125566 published on May 18, 1989).
In the other previously proposed ignition timing controlling system, a first correction coefficient is set such that the ignition timing is corrected according to the engine revolutional speed when the engine is in a transient operating state and a second correction coefficient is set such that the ignition timing is corrected on the basis of a load state of the engine, thus suppressing the engine rotational speed variations according to the load state during the engine acceleration/deceleration. Consequently, the generation of the shuddering vibrations of the vehicle is prevented.
However, the previously proposed ignition timing controlling system has drawbacks as follows:
A basic ignition timing angle is advanced when the engine revolutional speed is reduced during the generation of shuddering vibrations on a basis of a predetermined ignition timing angle map. On the other hand, when the engine revolutional speed is increased, the basic ignition timing is retarded. Therefore, when the basic ignition timing is set at an angle which exceeds a minimum angle for a best torque, so called, MBT due to an engine coolant temperature variation or aging effect on the engine, an optimum control for the shuddering vibrations cannot be achieved. in a worst case, a vibration attracting force is often impinged on the vibrations to assist the generation of the shuddering vibrations.